Resinous composition



. resins;

United States Patent C ice 2,948,705 RESINOUS COMPOSITION crimes Robinson, Woodhury, N.J., as signor to E. I. du Pont de Nemom's and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Feb. 1956, Ser. No. 564,633 2 Claims. :(Cl'. 260-47) The present invention relates to novel heat-resistant resinous compositions valuable as molding andpotting compositions, adhesives, laminates, and the like. More specifically, the present invention relates to epoxy resins cured by pyromellitic dianhydride-maleic anhydride mix tures.

2,948,705 Patented Aug. 9,

would beinsoluble in the resin. For example; German application- 7,193 describes the use as a during agent of pyro'melliti'c' dianhydride, apolyfunc't-ional high molecular-weight compound normally incompatiblein epoxy resins, by admixing' the pyromellitic dianhydi'ide' with suificien't phthali'c anhydride to produce-a mixture compatible with the epoxy resin. 7

The use of epoxy resins of many applications wherein the" composition must withstand relatively high tempera ttlre's been limited, however, because neither the acid and/or anhy'drid'e curing agents or the replaceable hydrogen containing curing agents known up to this invention were capable of producing a composition resistant to softening and to shrinking due to lo'ss'of weight upon exposure to high temperatures forprolonged periods of time. Compositions having relatively high heat 'dis'- tortion temperatures and minimal weight losses upon exposure to high temperature are ncededin many applications, such as in motors, electrical appliances, comrnunication devices, etc. j

An object of the present invention, therefore, is the prodnction of new compositions of matter valuable for the molecular weights of the individualpoiyethersof the mixture), the viscosity-, the melting point, and the average I number of 1,2-epoxide groups per unit weight of the epoxy resin depend on the ratio and the properties of the starting materials. Hence, epoxy resins may be: liquids or solids and may have molecular weights of 400 to 2000.

Although these high-'molecular-weight complex polyether compositions are thermoplastic, they arecapable of further reaction through the hydroxy' and/or 1,2 epoxide groups to form thermosett'n'g' compositions having outstandingpropertiesof adhesion, strength, and chemical resistance. The conditions under which the polyether composition is converted to' the thermosetting form and the physical and chemical: properties oi the finalcom position depend on the agent-through which the conversion; in; cure, 1s efiectedz' Since 1,2-epoxide groups are capable of reacting with hydroxy groups,cure may be byfldirect' reaction of the curing agent with the reac ,ve groups of the polyethericomposition or liy an ionic mechanism which catalyzes the inter and intramolecular reaction of the lgz-epoxide amt the hydroxy groups of the polyether 'coinpositiom' Suitable agents include aliphatic and aromatic acidsnan'd anhydi-i'des and compositions which? contain easily replaceablehydro'gens, such' as are available in primary and secondary ammesan amides arid urea -formaldchyde Thechoice: of curing 'agentialfects both the handling characteristics and the physical-1 properties of lthefinal composition? For: exampleaorganic acids and/or acid; 'anhydridee greatly im-thein solubilitiesinthe epoxyresins, 1 in: their" ability to' effect a curc 'in the time which a cure is eflfectedj. and in the properties they produce in the final: composition;

The; use of combinations of various organic acids arid/or their'anhydride'sasi curing-1 agcntsfon epoxy resins iswell known. The combinations: are usually formulated orf asliasisof' obtainingtin the final compositiona blending of the physica'lproperties ascriba ble toleacln of. the components ofthe curing mixturer However, such combma time alsomayhe preparedlinorder to permit incorpora- I, tion into the epoxy resin of a compound which otherwise loss or. weight upon prolonged exposure to liigh--tem I have found that the foregoing objects maybe attained when I provide a composition of an epoxy resin and a mixture of p'yromellit ic dianhydride andmal'cic anhydride, the amount of the anhydride rr'lixtur'e present heing' such as to provide from 0.7 to 0.9 anhydrideigroup for each epoxy group inthe epoxy resin. In my preferred composition, the pyromellitic dianhydride will provide from 25 to 65 of the anhydride groups;

The composition of my inventionmay readily be pre- "pared by heating the epoxy resin to a temperatureof from to C., gradually adding the mixture of pyrofnelliti'c dianhydride and maleic" anhydride to lt he molten epoxy resin, and thereafter heating the composition thus formed at frorrf 1 20 to 180 C.

A cured resin composition prepared as described above has anunusually good resistance to softening or loss-of weight due to hightemperature. The softening of a resin composition is usually determined by preparing a standard size bar of the composition and subjecting the bar to a fiber stress of 264' pounds per square inch. 'The temperature atwhich sufiicient softening has occurred to -permit. a' deflection of 10 mils is' known as the heat distortion temperature. The effect of high temperatures on weight loss of a resin is determined by subjecting the cured resin to a temperature of 200 for- SOOhours and measuring the loss in weight resulting therefrom:

The following examples illustrate the use or nurtures o nyromellitie dia'n'hydride' and m'aleic anhydride p'roduce cured epoxy resincompositions" inaccordancewith this invention". Ineach ase, a liquid epoxy resin which had an ap roximate molecular wei ht of 450, a softenin'g point of 8 C; and ane oxidev lue of 0:46 was sed.

Example 1 One hundred parts of the epoxy resin was heated to 9042050; and 39,6parts of a mire-near zo psra r pyromelliticdianhydride and 1916 par-ts of maleieanhy- 'd'riife was added" gradually, with stirring, to theiiicl't. When all theanhydridem'ixt'ure was in'sollltion',fthe coinith 221 hours at 0; A hard; toug'hgsolid casting, V which had aheat distortiomtemperatureos 2.10" C.-;;was I position was poured mm a mold and heated-inan oven 3 f obtained. When the same uncured composition was heated for 4 hours at 120 C. and then 20 hours at 160 C., a hard, tough, solid casting, which had a heat distortron temperature of 250 C., was obtained. When a cast-c ing of this composition was heated'for 500 hours'at 200 C.,a weight loss of only. 2.26% occurred. A weight loss of 25.6% under the above test conditions occurred when a phthalic anhydride-pyromellitic dianhydride mixture was used in lieu of the maleic anhydride-pyromellitic dianhydride mixture as the curring agent. In order to provide a substantially equivalent composition with respect to anhydride group ratios, 48 parts of a mixture of 20 parts of pyromellitic dianhydride and 28 parts of phthalic anhydride was used per hundred parts of resin.

In a similar run, in .which 23.3 parts of the mixture of 11.8 parts of pyromellitic dianhydride and 11.5 parts of maleic anhydride was mixed with 100 parts of the molten epoxy resin, the casting had a heat distortion temperature of only 101 C.

Example 2 One hundred parts of the epoxy resin was heated to 90-120 C. and 40.6 parts of a mixture of 18 parts of pyromellitic dianhydride and 23.6 parts of maleic am hydride was added gradually, with stirring, to the melt. When all of the anhydride mixture was in solution, the composition could be maintained at 90 C. for 5 6 minutes without the composition becoming too thick to pour. When the composition was poured into a mold and heated for 24 hours at 160 C.-, a hard, tough, solid casting, which had a heat distortion temperature of 227 C., was obtained. When a casting of this composition was heated 4 into a mold and heated in an oven at 120 C. for 4 hours and 160 C. for 20 hours. At the end of this time, a hard, tough, solid casting, which had a heat distortion temperature of 230 C., was obtained.

Example 7 One hundred parts of the epoxy resin was heated to 90-120 C. and 42.4 parts of, a mixture-of 21.6 parts of pyromellitic dianhydride and 20.8 parts of maleic anhydride was added gradually, with stining, to the melt. When all of the anhydride mixture was in solution, the composition was poured into a mold and heated in an ovenat 120? C. for 4hours and 160 C. for 20 hours.- At the end of this time, a hard, tough, solid casting, which broke at 216 C. when heated 'under'a fiber stress of 264 pounds per square inch, was obtained.

Example 8 One hundred parts of the epoxy resin was heated to 90l20 C. and 40parts of a mixture of 10.6, parts of pyromellitic dianhydride and 29.4 parts of maleic anhydridewas gradually added, with stirring, to the melt. When all of the anhydride mixture was in solution, the

composition was poured into a mold and heated in an for 500 hours at 200 C., a weight loss of only 2.55% 00- curred.

Example 3 One hundred parts of epoxy resin was heated to 90120 C. and 38.3 parts of a mixture of 14.8 parts of pyromellitic dianhydride and 23.4 parts of maleic anhydride was added gradually, with stirring, to the melt. When all the anhydride mixture was in solution, the composition was poured into a mold and heated in an oven at 160 C. for 24 hours. A hard, tough, solid casting, which had a heat distortion temperature of 7A3 C., was obtained. When a casting of this composition was heated for 500 hours at 200 C., a weight loss of only 2.85% occurred.

Example 4 One hundred parts of the epoxy resin was heated to 90-120 C. and 39.6 parts of a mixture of 20 parts of pyromellitic dianhydride and 19.6 parts of maleic anhydride was added gradually, with stirring, to themelt. When all the anhydiide mixture was in solution, the composition was poured into a mold and heated for 20 hours at 160 C. A hard, tough, solid casting, which had a heat distortion temperature of 265 C., was obtained.

Example 5 Example 6 One hundred parts of the epoxy resin was heated to 90120 C. and 35.2 parts of a mixture of 18 parts of pyromellitic dianhydride and 17.2 parts of maleic anhydride was added graduallyQwith stirring, to the melt. This mixture could be kept at 90 C. for 14 minutes before it became too thick to pour. When all of the anhydride mixture was in solution, the composition was poured oven for 4 hours at C. and 20 hours at C. A hard, tough, solid casting, which had a heat distortion temperature of C., was obtained. i I V The foregoing examples illustrate the high heat distortion temperatures and the resistance to weight loss at elevated temperature obtained when epoxy resins are cured by mixtures of pyromellitic dianhydride and maleic anhydride and also the proportions of the anhydrides in the mixture and the anhydride'epoxide ratios which will produce such compositions. The high heat distortion temperatures and lack of weight loss apparently are due'to the great density of cross-linkages in the cured resin which are produced by the compact polyfunctional pyromellitic dianhydride. The heat distortion temperature and the weight loss appear to depend on the amount of pyromellitic dianhydride in the final composition. The maleic anhydride, however, serves an extremely useful function,

not only as a flux for the pyromellitic dianhydride, but also as a compact curing agent in its own right, Although only a sutficient amount of maleic anhydride-to provide 35% of the anhydride groups in the mixture is required to insure compatability of the *anhydride mixture with the epoxy resin melt, up to 75% of the anhydride groups may be provided by this monoanhydride. Such larger proportions of maleic anhydride are especially advantageous if the anhydride-resin composition is not to be poured immediately. Thus, the composition of Example 2 could be retained in a. moltenco ndition longer thanthat of Example 4, even though the ratio of anhydride to epoxy groups in the final mixture of Example 2 was greater;

The amount of pyromellitic dianhydride in the final composition may be increased either by increasing the amount of pyromellitic dianhydride in the anhydride mixanhydride must be added that there is at least 0.7 anhydride group for each epoxide group of the epoxy resin. Thus, as shown in the second'portion of Example 1, when the amount of the enhydride mixture in the final composition was decreased to one which provided 0.5 anhydride group per epoxidegroup of the resin, the-heat distortion temperature of the cured composition was markedly lower than that of the composition in which the amount of anhydride mixture one which provided 0.7 anhydride group per epoxide group. When the pro portion of pyromellitic dianhydn'de in the mixture was decreased, as in Example 6 in which only 25% of the anhydride groups were provided by pyromellitic 'dianhydride, and the amount of anhydride mixture added was sufficiently high, a cured composition having a relatively high heat distortion temperature was obtained. I have found that compositions having high heat distortion temperatures will be obtained if mixtures of pyromellitic dianhydride and maleic anhydride in which from 25 to 65% of the anhydride groups are provided by the dianhydride are added to molten epoxy resins in amounts such that there is from 0.9 to 0.7 anhydride group for each epoxide group of the epoxy resin.

The mixed anhydride-epoxy resin compositions of the present invention were converted to compositions having the high heat distortion temperatures by heating at elevated temperatures for 24 hours. The curing may be efiected either by heating the composition at a single temperature or by heating the composition in stages, i.e. heating for several hours at a lower temperature, followed by a longer heating period at more elevated temperature. There is essentially no diiferenoe in the properties of single-stage and two-stage cured compositions at the optimum anhydride/epoxide ratio.

In order for the epoxy resin to be converted to a highmolecular-weight thermosetting composition by reaction of the curing agent with the 1,2-epoxide group, the resin must have a 1,2-epoxide equivalency greater than one. By epoxide equivalency is meant the average number of 1,2-epoxide groups contained in the measured molecular weight of the resin. Since the epoxy resin is a mixture of polyethers, the measured molecular weight, upon which the 1,2-epoxide equivalency depends, is necessarily an average molecular weight. Hence, the 1,2-epoxide equivalency of the resin will not necessarily be a whole number, but will be a number greater than one. The 1,2-epoxide equivalency can be determined if the measured molecular weight and the epoxide value are known. Thus, an epoxy resin having an average molecular weight of 900 and an epoxide value of 0.2 has a 1,2-epoxide equivalency of 1.8.

The epoxide value of an epoxy resin is the number of epoxide groups per 100 grams of resin. This value can be determined experimentally by heating a one-gram sample of the epoxy resin with an excess of a pyridine solution of pyridine hydrochloride (obtained by adding sixteen cubic centimeters of concentrated hydrochloric acid to a liter of pyridine) at the boiling point tor twenty minutes and then back-titrating the unreacted pyridine hydrochloride with 0.11 N sodium hydroxide to the phenolphthalein end point. In the calculations, each H01 consumed by the resin is considered to be equivalent to one epoxi'de group.

Although the invention has been described in detail in the foregoing description, it will be apparent that many variations can be made without departing from the basic concept or the invention. I intend, therefore, to be limited only by the following claims.

I claim:

1. Composition of matter comprising an epoxy resin selected from the group consisting of glycidyl polyethers of polyhydric alcohols and glycidyl polyethers of polyhydric phenols and having a 1,2-epoxide equivalency greater than one and a mixture of pyromellitic dianhydride land maleic anhydride in which firom 25 to of the anhydride groups of the anhydride mixture are contributed by the pyromelliti'c dianhydride, the amount of the anhydride mixture being that which provides 0.7 to 0.9 anhydride group for each epoxide group of said epoxy resin.

2. A process for producing novel resinous compositions comprising heating an epoxy resin selected :from the group consisting of glycidyl polyethers of polyhydric alcohols and glycidyl polyethers of polyhydric phenols and having a 1,2-epoxide equivalency greater than one to -120 C., gradually adding to the molten resin an amount of a mixture of pyromellitic dianhydride and maleic anhydride such that there are 0.7 to 0.9- part of anhydride for each epoxy group of the epoxy resin, from 25 to 65% of the anhydride groups of the anhydride mixture being contributed by the pyromellitic dianhydride, and heating the clear composition at a temperature of from to C.

References Cited in the file of this patent UNITED STATES PATENTS Castan July 20, 1943 Carey June 17, 1958 

1. COMPOSITION OF MATTER COMPRISING AN EPOXY RESIN SELECTED FROM THE GROUP CONSISTING OF GLYCIDYL POLYETHERS OF POLYHYDRIC ALCOHOLS AND GLYCIDYL POLYETHERS OF POLYHYDRIC PHENOLS AND HAVING A 1,2-EPOXIDE EQUIVALENCY GREATER THAN ONE AND A MIXTURE OF PYROMELLITIC DIANHYDRIDE AND MALEIC ANHYDRIDE IN WHICH FROM 25 TO 65% OF THE ANHYDRIDE GROUPS OF THE ANHYDRIDE MIXTURE ARE CONTRIBUTED BY THE PYROMELLITIC DIANHYDRIDE, THE AMOUNT OF THE ANHYDRIDE MIXTURE BEING THAT WHICH PROVIDES 0.7 TO 0.9 ANHYDRIDE GROUP FOR EACH EPOXIDE GROUP OF SAID EPOXY RESIN 